1. Field of the Invention
The present invention relates to a clamp circuit for a solid-state imaging device and, more particularly, to a clamp circuit which clamps a reference black level in a video signal, at a signal level which is obtained when an optical black portion is read.
2. Description of the Prior Art
In a solid-state imaging device such as a CCD, a reference black level is set so that an output video signal always has a brightness which corresponds to the reference black level. As shown in U.S. Pat. No. 3,806,729 which issued on Apr. 23, 1974 to J. M. Caywood, in order to obtain the reference black level, an optical black light-shielded portion is formed outside of an effective scan area of an imaging surface. The level of video signal obtained when the optical black portion is scanned is used as the reference level to indicate a black reference level in the video signal.
When the reference black level is clamped as described above, if light energy is overincident on the imaging surface, then mis-clamping occurs. More specifically, if an emitter such as the sun, a lamp, or the like emits high intensity light relatively near the optical black portion, such light leaks around the object. Some of such leaking light energy is incident below the optical black portion, and is photoelectrically converted into an electrical signal. If such phenomenon occurs, the level of the video signal obtained when the optical black portion is scanned drifts from a level corresponding to a reference black, which results in the occurance of mis-clamping.